INDEPENDENT MENDELIAN INHERITANCH $1 
These two cases illustrate very well how closely the results of Men- 
delian investigations fulfill mathematical requirements, and their signifi- 
cance cannot be doubted when it is considered how little difficulty is 
experienced in classifying this particular kind of material. Nevertheless 
the mathematical requirements are very often not fulfilled on account 
of the action of external conditions of various kinds. Here as elsewhere 
the disturbing influence of biological factors must ever be kept in mind 
in judging the significance of the application of any strict mathematical 
tests. 
Dihybridism.—When two pairs of factor differences are involved in 
a hybrid the same laws apply in segregation and recombination as apply 
in the monohybrid. The two pairs of factors segregate independently 
Fig. 38.—Maize ear showing F2 segregation of grains in the ratio of 3 purple, 1 white. 
of each other and give character combinations in ’, to be expected on the 
basis of chance factor distribution. In maize there are varieties which 
have a deep purple aleurone color which gives the entire grain a black 
appearance. When such varieties are crossed with certain white varie- 
ties which possess no aleurone color the F; is purple and in 2 the grains 
are in the ratio of 3 purple: 1 white. An ear displaying such F’2 segrega- 
tion is shown in Fig. 38. The factors involved in this case are W for 
pigment production in the aleurone layer and w for no pigment. pro- 
duction in this tissue. The hybrid Ww since it is a monohybrid will, 
therefore, give in F', genotypes in the ratio 1 WW :2 Ww :1 ww, which are 
distributed in two phenotypes in the ratio 3 purple: 1 white. We have 
shown similarly how starchy corn when crossed with sweet gives a starchy 
F, and in F, 3 starchy:1 sweet. Here the factors involved are S for 
starchiness and s for sweet. A purple sweet corn, therefore, will have 
the genetic constitution WWss with respect to the above factors, and a 
white starchy corn, the genetic constitution wwSS. 
When a purple sweet corn is crossed with a white starchy corn the 
F, will be purple starchy—it will display the dominant characters of both 
parents to the exclusion of the recessive characters, white and sweet. 
From the purple sweet corn the F; receives gametes of the genetic consti- 
tution Ws and from the white starchy wS. Consequently its genetic 
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